Television transmitter switching apparatus



Decyz, I195@ E. L. c. WHITE ET AL 2,535,471

TELEVISION TRANSMITTER SWITCHING APPARATUS Filed Feb. l2. 1947 ERIC L. C'. WHITE FRANK R. TROTT I47' 7' ORNE Y Patented Dec. 26, 1950 UNITED STATES PATENT OFFICE TELEVISION TRANSMITTER SWITCI-IING APPARATUS Application February 12, 1947, Serial No. 728,120 In Great Britain January 1946` Section 1, Public Law 690, August 8, 1946 Patent expires January 15, 1966 5 Claims.

This invention relates to television' apparatus.

In transmitting television programmesv it is frequently desiredv to change from one scene to another and for this purpose' the output of one Jtelevision` camera is switched from the transmitter and replaced by the output from another camera. Usually such change is effected by fading out the outputfrom one camera and by fading in the output from. the other camera. In some cases, however, it may be desired to effect an' instantaneous switch from one camera to another, but heretofore such an instantaneous switchr has usually resulted in an unsightly splash on the screen. ofA a television receiver owing tothe fact that momentarily one part of a frame. scan. will appear to be occupied by the scene from one camera and the remainderby the scene from the other camera. Similar difficulties also arise in cases where it may be desired to vary the level of television signals, in discrete steps.

The: object of the present invention is to provideA improved apparatus whereby suchr rapid changes in. television signals can bey effected.

According to the present invention there is provided television, apparatus wherein means are provided whereby rapid changes inv television signals; such asan instantaneous switch fromrone camerato another or a rapidv change in the level of said signals. can only be eiected in the inter-vals between successive frame scans.

rThe apparatus may comprise a switching device for effecting said rapid changes, means under the control of. an operator for controlling theoperation of said switching device, and' further' means for effecting a further control on said switching device,v said further means being synchronised with said intervals whereby said switching device can only be operated tov cause said changeswhen said further control is effected. n

In order that the said inventionl may be clearly understood and readily carried into effect, it will now be more fullyd'escribe'd with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic view of a mechanical commutator device for use in carrying out the invention,

Figure 2 is a circuit diagram of a thermionic valvearrangement f'or use in place of the cornmutator device shown in Figure 1,

Figure 3 illustrates the waveform of pulsesr suitable: for' application to the arrangement shown in Figure 2,

Figure 4 isA a circuit diagram illustrating an arrangement in` which the mechanical switches shown` in Figure 1 are: replaced by thermionic valvti'SQ-and'y 2 Figure 5 isa modification of the arrangement shown in Figure 4.

In Figure l of the drawings the output from one television camera may be fed along a channel I and the output from another camera along the channel 2', the channels I and 2 being alternately switched under the control of the operator to feed the output fromv either channel I or 2 to the signal output channelV 3. Switching of the channel IA or 2 to the channel 3 is eiiected in Figure l by the operation of a contact 4 which is controlled by a relay 5 which causes said contact 4 to engage either of a pair of contacts 6 and 'I connected respectively to channels I and 2. The' relayv 5 is energi-sed by' a source of current indicated at 8", the circuit to the relay 5 being closed via a pair of commutators 9 and I0 under the control of a manually-operated key Il. The commutators 9 and I0 are mounted on a shaft I2 which is driven synchronously at the frame frequency and the comuntator 9 is provided with a conducting segment I3 so that the commutator 9` can onlymake' contact with its associated brush I 4V during' the intervals between successive frame scans, whilst the commutator I0 is provided with an insulated segment I5 so that it makes contact with its associated brush I8 at all times except during the intervals between successive frame scans. The brush I6 is arranged in series with relay-holding contacts I'I whereby the relay 5 can beheld energised when the brush I6 engages the conducting portion of the commutator ID. Assuming that the commutators are in the position shown in Figure 1 and assuming' that the key I I is depressed in order to effect an instantaneous switch from one' camera to another, the relay 5 will not operate to change the position of the` contact 4 until the commutator 9 has rotated to a position in which the conducting segment I3 makes contact with the brush I4, that is to say, until the interval which occurs after the frame scan during4 which the key II was depressed. When the conducting segment I3 moves out of engagement with the brush I4 the relay 5 will remain in its energised condition through current ,supplied to the relay from the brush I6 and the conducting portion of the commutatorl I0. When itv is desired to effect a further instantaneous switch the key Il is opened and the relay 5 will remain in its energised position through the selfhold contacts I'I until the non-conducting segment I5 again engages the brush I6 when the relay 5 will be de-energisedY The mechanical commutators' shown in Figure l may be replaced by' a pair of thermionic valves, asshowniu the circuit of Figure 2 In thisgure 3 a pair of thermionic valves i9 and 2i) are provided, the cathodes ci which are coupled together and to the negative pole of a source of supply via a resistance 2l whilst the anode of the valve I9 is connected to the control electrode of the valve 2e as indicated, the arrangement forming in known manner a so-called flip-flop circuit which has two stable conditions in one of which the valve iii is conducting and in the other of which the valve 23 is conducting. The relay 5 is arranged in the anode circuit of the valve 2) and current is supplied to the valves as conventionally indicated by the batteries shown in Figure 2. With a circuit as shown in Figure 2, a change from one condition to the other can be effected by varying the bias potential applied to the control electrode of the valve is until it reaches a critical potential. With a circuit as shown in Figure 2, there are two critical potentials corresponding to the two stable conditions. The bias potential of the control electrode of the valve i9 is changed by operating the key i l which serves to connect the positive cr negative end of a bias battery 22 to the control electrode of the valve iS according to the position of the key il. The change of p0- tential applied tc the control electrode of Valve I9 on operation of the key I! is arranged to be less than that required to change the condition of the circuit from one state to the other and, in addition to this change in potential, pulses are applied to` the control electrode of the valve I9, these pulses occurring during the intervals lbetween successive frame scans so that if the key li is operated a 'change in stable condition of the valves IS and 2Q will not be eiected until a pulse is applied to the control electrode of the valve i9. Referring to Figure 3 of the drawings, the dotted lines 23 and 2li indicate the biasl potentials applied to the control electrode of the valve I9I from the battery 22 when the key II is operated, these bias potentials being restricted to, say, half the diierence between the two critical potentials E1 and E2 so that if merely the key I I were operated without pulses being present on the control electrode of the valve i9, no change in the stable The waveform of pulses condition would occur. suitable for application to the control electrode of the valve I9 is indicated by full lines in Figures 3, these pulses comprising positive portions 25 and negative portions 25 each of which has an amplitude substantially equal to half the difference.

between the critical lpotentials, so that if the key l i is operated then `a change in the stable condition of the valves IQ and 253 will occur immediately following the application of either a a positive or negative pulse depending on the position of the key i i. W ith the key I I in the position shown in Figure 2, in which position a negative biasv is applied to the control electrode rof `the valve Iii, the valves i9 and 29 will change from one state to the other when the firstnegative pulse occurs and with the key II depressed in up devices one or the other of which is connectedA to the output channel by operation of relay 5 on switch number 5. The rectangle indicated by the letter'Y CA represents a conventional apparatus for production of rectangular waves.

The positive and negative pulses 25 and 25V' should occur closely following one another leaving. aimaient, time .aitergltlicrenterrement, 15.1.1@

second of the pair of pulses for the relay 5 to operate before the end of the interval between successive frame scans. The pulses 25 and 26 may be produce, for example, by diierentiating a single short rectangular pulse by feeding said rectangular pulses to a diierentiating circuit comprising condenser 2l and resistance 28 connected to the control electrode of the Valve I9, the time constant of the condenser 2'I and resistance 28 being short compared with the duration of each rectangular pulse. 1f desired, instead of providing said positive and negative pulses closely following one another, positive and negative pulses may be employed which occur alternately in the intervals between successive frame scans.

Instead of employing the relay 5 ior effecting switching from one camera channel to the other, the relay 5 may be replaced by two pairs of diode valves 2S, Bil and 3l, 32 as indicated in Figure 4. The camera channels I and 2 are connected to the anode of diode 29 and to the anode of diode 3i respectively, the diodes operating to switch the output of either channel I or 2 to the signal output channel 3 in a manner described in the specification of British Patent No. 497,637. The arrangement in Figure 4 is suitable for feeding the signal output to a high impedance load, such as the control electrode of a subsequent valve. if it is desired to feed the signal output to a, low impedance load, then the diode valves 2S, 3f) and 3i, 32 may be replaced by pairs of triode or pentode valves '33, 3ft and 35', 36 as shown in Figure 5. The valves 33, 34 and 35, 33 form part ci an amplier circuit the output from which is fed into a cathode follower valve 31 as described in the specification of British Patent No. 564,821.

In Figure 4 the diode valves 29 32 are coupled as shown to the anodes of a pair of cathode-coupled triodes 33 and 39, the control electrode of the valve 33 being 'connected to the anode of the valve i9 of Figure 2 so that the valves 33 and 39 will operate when a change in the stable condition of the valves I9 and 20 occurs. Similarly, in Figure 5 of the drawings, the cathodes of the pentode valves 33 36 are coupled to the anodes of a pair of cathode-coupled pen- ,v tode valves iii and 3 i, the control electrode of the operated synchronously to simultaneously scan.

target electrodes contained therein in a succession of frame scanning cycles and a single transmission channei for the transmission of only one of the produced plurality of video signals at any time, comprising switching means for selectively switchingoperation tointergals between successive frame scanning cycles.A

2. A television transmission system lincluding a plurality of sources of ytelevision video signal trains, eachsource, supplyingwvideo signals interspersed with concomitantly occurring blanlniny.

tervals, a single transmission channel, means for applying one or another of the video signal trains to the transmission channel comprising a manually operated initiating circuit and a final switching circuit, and means for actuating the nal switching circuit during the next succeeding blanking interval following the instant of operation of the initiating circuit.

3. A switching circuit for connecting a single television transmission channel to one or another of a plurality of synchronously operated television cameras comprising a manually operated selection means for selecting the desired camera, a source of recurring impulses related to the synchronous operation of the cameras, and means responsive to the selection means and the recurring impulses to effect final switching of the cameras only during the vertical blanking periods of the scanning cycles.

4. A television transmission system including a plurality of sources of television video signal trains, each source supplying video signals interspersed with concomitantly occurring vertical blanking intervals, a single transmission channel, means for applying one or another of the video signal trains to the transmission channel comprising a thermionic switching circuit having two stable conditions, means for changing a control potential under the control of an operator from either one or the other of two values to condition the switching circuit for operation, a series of impulses each occurring substantially simultaneously with the concomitant blanking intervals, and means responsive to the next succeeding impulse following the manual conditioning of the switching circuit for effecting a nal switching.

5. In a television system having a plurality of video pick-up devices, a switch for selectively applying the output of any one of said pick-up devices to a common transmission channel comprising a device having a plurality of operative positions, circuit arrangements for connecting the output of one of said pick-up devices to said channel in each position, means for selectively conditioning said device for a change from one operative position to a predetermined operative position, means for periodically blanking the output of each of said pick-up devices, and means for changing said device from said one operative position to the predetermined operative position for which it was conditioned during the time the pick-up devices connected to the initial position and the predetermined position are blanked out.

ERIC LAWRENCE CASLING WHITE. FRANK RUNCORN TROTT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,870,352 Watson Aug. 9, 1932 2,000,694 Felix May 7, 1935 2,107,464 Zworykin Feb. 8, 1938 2,116,549 Zenner May 10, 1938 2,359,449 Shelby Oct. 3, 1944 FOREIGN PATENTS Number Country Date 325,362 Great Britain Feb. 20, 1930 

